* Man-in-the-middle attack *

حمله مرد میانی

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2012)

The man-in-the-middle attack (often abbreviated MITM, MitM, MIM, MiM, MITMA) in cryptography and computer security is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that they are talking directly to each other over a private connection, when in fact the entire conversation is controlled by the attacker. The attacker must be able to intercept all messages going between the two victims and inject new ones, which is straightforward in many circumstances (for example, an attacker within reception range of an unencrypted Wi-Fi wireless access point, can insert himself as a man-in-the-middle).

A man-in-the-middle attack can succeed only when the attacker can impersonate each endpoint to the satisfaction of the other—it is an attack on mutual authentication (or lack thereof). Most cryptographic protocols include some form of endpoint authentication specifically to prevent MITM attacks. For example, SSL can authenticate one or both parties using a mutually trusted certification authority.

All cryptographic systems that are secure against MITM attacks require an additional exchange or transmission of information over some kind of secure channel. Many key agreement methods have been developed, with different security requirements for the secure channel. Interlock Protocol attempts to address this.

Example of an attackIllustration of man-in-the-middle attack.

Suppose Alice wishes to communicate with Bob. Meanwhile, Mallory wishes to intercept the conversation to eavesdrop and possibly (although this step is unnecessary) deliver a false message to Bob.

First, Alice asks Bob for his public key. If Bob sends his public key to Alice, but Mallory is able to intercept it, a man-in-the-middle attack can begin. Mallory sends a forged message to Alice that claims to be from Bob, but instead includes Mallory''s public key.

Alice, believing this public key to be Bob''s, encrypts her message with Mallory''s key and sends the enciphered message back to Bob. Mallory again intercepts, deciphers the message using her private key, possibly alters it if she wants, and re-enciphers it using the public key Bob originally sent to Alice. When Bob receives the newly enciphered message, he believes it came from Alice.

1. Alice sends a message to Bob, which is intercepted by Mallory:

Alice "Hi Bob, it''s Alice. Give me your key"--> Mallory Bob

2. Mallory relays this message to Bob; Bob cannot tell it is not really from Alice:

Alice Mallory "Hi Bob, it''s Alice. Give me your key"--> Bob

3. Bob responds with his encryption key:

Alice Mallory <-- Bob

4. Mallory replaces Bob''s key with her own, and relays this to Alice, claiming that it is Bob''s key:

Alice <-- Mallory Bob

5. Alice encrypts a message with what she believes to be Bob''s key, thinking that only Bob can read it:

Alice "Meet me at the bus stop!"--> Mallory Bob

6. However, because it was actually encrypted with Mallory''s key, Mallory can decrypt it, read it, modify it (if desired), re-encrypt with Bob''s key, and forward it to Bob:

Alice Mallory "Meet me in the windowless van on 22nd Ave!"--> Bob

7. Bob thinks that this message is a secure communication from Alice.

This example shows the need for Alice and Bob to have some way to ensure that they are truly using each other''s public keys, rather than the public key of an attacker. Otherwise, such attacks are generally possible, in principle, against any message sent using public-key technology. Fortunately, there are a variety of techniques that help defend against MITM attacks.

Defenses against the attack

This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2010)

Various defenses against MITM attacks use authentication techniques that include:

PKI mutual authentication The main defence in a PKI scenario is mutual authentication. In this case as well as the application validating the user (not much use if the application is rogue)—the users devices validates the application—hence distinguishing rogue applications from genuine applications

A recorded media attestment (assuming that the user''s identity can be recognized from the recording), which can either be:

A verbal communication of a shared value for each session (as in ZRTP)

An audio/visual communication of the public key hash (which can be easily distributed via PKI)

Stronger mutual authentication, such as:

Secret keys (which are usually high information entropy secrets, and thus more secure), or

Passwords (which are usually low information entropy secrets, and thus less secure)

Latency examination, such as with long cryptographic hash function calculations that lead into tens of seconds; if both parties take 20 seconds normally, and the calculation takes 60 seconds to reach each party, this can indicate a third party

Second (secure) channel verification

Carry-forward verification

Testing is being carried out on deleting compromised certificates from issuing authorities on the actual computers and compromised certificates are being exported to sandbox area before removal for analysis

The integrity of public keys must generally be assured in some manner, but need not be secret. Passwords and shared secret keys have the additional secrecy requirement. Public keys can be verified by a certificate authority, whose public key is distributed through a secure channel (for example, with a web browser or OS installation). Public keys can also be verified by a web of trust that distributes public keys through a secure channel (for example by face-to-face meetings).

See key-agreement protocol for a classification of protocols that use various forms of keys and passwords to prevent man-in-the-middle attacks.

Forensic analysis of MITM attacks

Captured network traffic from what is suspected to be a MITM attack can be analyzed in order to determine if it really was a MITM attack or not. Important evidence to analyze when doing network forensics of a suspected SSL MITM attack include:

Do other clients, elsewhere on the Internet, also get the same certificate?

Quantum cryptography

Quantum cryptography protocols typically authenticate part or all of their classical communication with an unconditionally secure authentication scheme e.g. Wegman-Carter authentication.

Beyond cryptography

A notable non-cryptographic man-in-the-middle attack was perpetrated by a Belkin wireless network router in 2003. Periodically, it would take over an HTTP connection being routed through it: this would fail to pass the traffic on to destination, but instead itself respond as the intended server. The reply it sent, in place of the web page the user had requested, was an advertisement for another Belkin product. After an outcry from technically literate users, this ''feature'' was removed from later versions of the router''s firmware.

In 2013, the Nokia''s Xpress Browser was revealed to be decrypting HTTPS traffic on Nokia''s proxy servers, giving the company clear text access to its customers'' encrypted browser traffic. Nokia responded by saying that the content was not stored permanently, and that the company had organizational and technical measures to prevent access to private information.

Another example of a non-cryptographic man-in-the-middle attack is the "Turing porn farm". Brian Warner says this is a "conceivable attack" that spammers could use to defeat CAPTCHAs. The spammer sets up a pornographic web site where access requires that the user solves the CAPTCHAs in question.

However, Jeff Atwood points out that this attack is merely theoretical—there was no evidence by 2006 that any spammer had ever built such a system. However, it was reported in October 2007 that spammers had built a Windows game in which users are asked to interpret CAPTCHAs acquired from the Yahoo! webmail service, and are rewarded with pornographic pictures. This allows the spammers to create temporary free email accounts with which to send out spam.

Implementations

Cain and Abel – a Windows GUI tool which can perform MITM attacks, along with sniffing and ARP poisoning